Flexible loads and power electronic converters integration : operation and control

Abstract

Renewable energy generations, such as wind turbines and photovoltaic panels, are being increasingly installed to achieve sustainability of climate and energy. However, high penetration of renewable energy sources will increase the uncertainty of the power grid and challenges the power grid operation due to their intermittent and distributed nature. The control paradigm moves towards the demand side and controls the load consumption to balance generation and demand. Direct load control, time of use tariffs and real-time pricing are classical methods but can’t ensure high response from the demand side. More advanced methods are essential to encourage end users to be more responsive. Some controllable loads, such as water heaters, ventilation and air conditioning systems, are an attractive source of demand response. Various methods are proposed to control the load profile in a way good for the grid operation and reduce the impact on end users through reasonable scheduling of these controllable loads. Electric Spring (ES) has been proposed as a new demand side solution. ES is integrated with a controllable load to form a flexible load which can increase and decrease its load consumption and provides extra power compensation for ancillary services. Majority of existing research work focuses on different ESs and various applications but pays little attention to how much they can do. Inspired by the concept of Electric Spring, this thesis proposes and discusses three types of flexible loads integrated with power electronic converters. Firstly, Chapter 3 explores the operating range of flexible loads integrated with ES. Based on the power analysis, equations linking ES parameters (i.e. voltage, current and active power) and load tolerance to the power of flexible loads are derived. Using these equations, the power range of flexible load with ES can be calculated directly with consideration of ES specifications and load tolerance. This also provides a new power control method that only consists of a few simple calculations. Simulations are conducted to verify the effectiveness of the control method. Secondly, Chapter 4 proposes a new flexible load which is integrated with a common three-phase power converter. Compared to existing ESs, the three-phase converter gets rid of batteries and isolation transformer. Thus, its size and cost are reduced. To expand its operating range, an optimal operation strategy is developed and tested. Experiment agrees well with calculation and validates that the proposed flexible load using a three-phase converter maintains flexible active and reactive power compensation. It is effective as a demand response method. Finally, Chapter 5 proposes a flexible light-emitting diode (LED) lighting system. This flexible LED device can adjust its power consumption without disruptive impact on human visual comfort. To achieve this goal, a visual investigation is conducted to figure out the typical rate of change of LED power and it is embedded into a new power control method. A smart LED driver is built to validate the control method. Visual assessments confirm that the proposed control method can significantly reduce the light flickering caused by the change of LED power. In summary, this thesis synthesizes power analysis, operating range and power control method of three types of flexible loads to provide reliable solutions to demand response.